The present invention relates generally to the field of air separation and has particular reference to the crude recovery of at least one rare gas selected from the group consisting of krypton and xenon from an oxygen product of an air separation.
Krypton and xenon are present in air at very low concentrations, typically about 1.14 parts per million (“ppm”) and about 0.087 ppm respectively. They are both valuable gases and, thus, there is an economic incentive to maximise their recovery in an air separation process.
In typical cryogenic air distillation processes, krypton and xenon concentrate in the liquid oxygen (“LOX”) product taken from the bottom of the low pressure (“LP”) distillation column as they are far less volatile than oxygen. The smaller the LOX flow, therefore, the more concentrated the krypton and xenon in this product.
In cryogenic air distillation processes in which most of the oxygen product is removed from the LP column in the gas phase, it is possible to make sure that very little krypton and xenon is lost in the gaseous oxygen (“GOX”) by removing the GOX several distillation stages above the bottom of the LP column. These bottom guard stages are used mainly to prevent excessive loss of krypton which is substantially more volatile than xenon. Almost all of the krypton and xenon entering the air separation plant can then be recovered in the LOX product, which is a very small proportion of the total oxygen flow. This LOX product can then be processed to produce a purified rare gas product. In the event that it is primarily a xenon product that is required, one could dispense with the bottom guard stages and still recover much of the krypton and almost all of the xenon entering the plant in the LOX product.
If the LOX flow from the distillation process is much greater, for example when all the oxygen is withdrawn from the distillation column as LOX, pumped to the required pressure and evaporated in the main heat exchanger, the loss of krypton and xenon is much greater, even when the LOX is taken several stages up the LP column, separately from the liquid stream in which is concentrated the rare gas components. Essentially all of the krypton and xenon entering the air separation plant flows down the LP column to the sump of the LP column in the descending liquid, so any liquid withdrawal will remove a portion of the krypton and xenon proportional to the total liquid withdrawn as product. This will typically lead to losses of about 30% of these valuable products.
It is desirable, therefore, to increase the recovery of krypton and xenon from an air separation plant in which at least part of the oxygen product is withdrawn as LOX.
When a plant withdraws the main oxygen product as a vapor from the LP column, the krypton and xenon can be recovered by processing the LOX product as described above. However, for existing pumped LOX cycle plants, there is usually no small stream having concentrated rare gas components as all oxygen products are generally withdrawn from the bottom of the LP column. Therefore, as krypton and xenon are so valuable, it is also desirable to be able to retrofit rare gas recovery systems to existing plants.
In addition, it is desirable to provide a xenon and/or krypton recovery plant, which can process rare gas-enriched feed streams from an external source.
U.S. Pat. No. 4,805,412 (Colley; published on 21st Feb. 1989) discloses a process and apparatus for the cryogenic distillation of air with reduced loss of krypton and xenon. Oxygen is withdrawn from the LP column of the distillation system and is fed to a primary krypton column for extraction of its krypton and xenon content. The main feed to the primary krypton column is a stream of LOX but a small stream of GOX is also taken from the LP column and is fed without pressure adjustment to the krypton column. The LP column and the primary krypton column operate at substantially the same pressure. A portion of the krypton-lean overhead vapor is condensed and fed to the primary krypton column as descending wash liquid.
U.S. Pat. No. 6,301,929 (Lochner, published on 16th Oct. 2001) discloses an air separation process in which a rare gas-lean LOX stream and a rare gas-enriched LOX stream are formed. The two liquid streams are pumped to a rare gas separation column operating at GOX product pressure. The rare gas-lean LOX stream is passed as reflux to the top of the column and the rare gas-enriched stream is passed to a lower section of the column. Rare gas-lean GOX product is withdrawn as overhead from the column and a further rare gas-enriched bottoms liquid stream is withdrawn. The reboiler/condenser in the sump of the column is sized to vaporize almost all the oxygen feed streams. As the oxygen feed streams are liquid, the reboiler/condenser must be large to vaporize all of the feed.
Research Disclosure No. 42517 (disclosed anonymously in September 1999) discloses an air separation process in which the oxygen product is removed from the column system as LOX. The LOX stream is pumped to the oxygen product pressure and divided into two steams. The first stream is passed as reflux to the top of a rare gas column and the second stream is passed to a lower zone of the column. The relative proportions of the two streams are determined such that the column can reject methane. Rare gas-lean GOX product is withdrawn as overhead from the rare gas column and a rare gas-enriched bottoms liquid stream is withdrawn. The reboiler/condenser in the sump of the rare gas column must be sized to vaporize almost all the oxygen feed streams. As the oxygen feed streams are liquid, the reboiler/condenser must be large to vaporize all of the feed.
DE-A-19855485 (Lochner; published on 10th Jun. 1999) discloses an air separation process in which rare gas-lean LOX and rare gas-enriched LOX are formed in the LP column. The two liquid streams are pumped to a rare gas column, the lean stream being passed as reflux to the top of the column and the enriched stream being passed to a lower section of the column. In addition, some gaseous nitrogen (“GAN”) is added to the bottom of the rare gas column to strip liquid descending the column. Rare gas-lean GOX overhead is returned to the LP column and a further rare gas-enriched LOX stream is withdrawn.
U.S. Pat. No. 6,378,333 (Dray; published on 30th Apr. 2002) discloses an air separation process in which a first LOX stream having a xenon component is passed from the LP column to the upper portion of a xenon concentrator column as reflux. In the xenon concentrator column, the LOX feed is separated into xenon-rich bottoms liquid and xenon-lean GOX overhead. A second LOX stream having a xenon component is withdrawn from LP column, pressurized and partially vaporized against a portion of feed air. Typically, the liquid fraction from this partial vaporization will be also passed as feed to the xenon concentrator column.
U.S. Pat. No. 5,913,893 (Gary et al, published on 22nd Jun. 1999) discloses a method of purification of a cryogenic fluid, especially liquid helium, by filtration and/or adsorption. The impurities are filtered/adsorbed from the fluid but are not available as a valuable product.
U.S. Pat. No. 5,039,500 (Shino et al, published on 13th Aug. 1991) discloses the gasification of a small LOX purge stream taken from an air separation unit (“ASU”) and passing the gaseous stream through an adsorber which selectively adsorbs xenon. Xenon is recovered during the regeneration phase of the adsorber. The xenon concentration in the purge stream is about 31 ppm, i.e. about 360 times the concentration of xenon in air.
JP-A-09002808 (Takano et al; published on 7th Jan. 1997) discloses the gasification of a small LOX purge stream taken from an ASU and passing the gaseous stream through a first adsorber (which selectively adsorbs xenon) and then through a second adsorber (which selectively adsorbs krypton). Xenon and krypton are recovered during the regeneration phase of the adsorbers.
It is well known in the art that krypton and xenon will concentrate in oxygen liquid because of the extremely low volatility of these gases. Thus, it is a requirement in the prior art that a LOX stream be processed in order to recovery krypton and xenon. Most of the prior art additionally provides a small oxygen purge stream concentrated in krypton and xenon so that the crude recovery system will be smaller. There is no disclosure in the aforementioned prior art of the recovery of krypton and xenon from a warm product gaseous oxygen stream.
In prior art processes, if krypton and xenon recovery is required, it is generally necessary to design the LP column of an ASU so that a small rare gas-rich LOX purge can be withdrawn. Such modifications add significantly to the necessary capital investment and to the height to the LP column.
It is desirable to overcome disadvantages of (and thereby improve on) the exemplified prior art and to provide an air separation process that is able to produce a rare gas-enriched product (for further processing into purified krypton and/or xenon products) and a pure LOX product without involving the capital expense and running costs of a large reboiler/condenser or additional equipment such as an argon stripping column.